js 如何在数字前面自动补零，生成序列号、单据号

  //获取指定位数的序号（如：003）
  var GetSerialNum = function(num,length){
     var serialNum = (Array(length).join(0) + num).slice(-length);
     console.log(serialNum);
     return serialNum;
 }
 
 //获取指定日期格式的日期
 var getFullDataStr = function (dateformat) {
     var dateStr = "";
     var mydate = new Date();
     var year = mydate.getFullYear();  //console.log(typeof(year));  可输出值的类型
     var month = (mydate.getMonth() + 1) < 10 ? '0' + (mydate.getMonth() + 1) : (mydate.getMonth() + 1) + '';
     var day = mydate.getDate() < 10 ? '0' + mydate.getDate() : mydate.getDate() + '';
     var hours = mydate.getHours() < 10 ? '0' + mydate.getHours() : mydate.getHours() + '';
     var minutes = mydate.getMinutes() < 10 ? '0' + mydate.getMinutes() : mydate.getMinutes() + '';
     var seconds = mydate.getSeconds() < 10 ? '0' + mydate.getSeconds() : mydate.getSeconds() + '';
     //var minseconds = mydate.getMilliseconds().toString().substring(0,2);
     var minseconds = mydate.getMilliseconds().toString();
     if (dateformat.toLowerCase() == "yyyy") {
         dateStr = year;
     }
     else if (dateformat.toLowerCase() == "yyyymm") {
         dateStr = year + month;
     }
     else if (dateformat.toLowerCase() == "yyyymmdd") {
         dateStr = year + month + day;
     }
     else if (dateformat.toLowerCase() == "yyyymmddhhmmss") {
         dateStr = year + month + day + hours + minutes + seconds;
     }
     else if (dateformat.toLowerCase() == "yymmddhhmmss") {
         dateStr = year.toString().substring(2, 4) + month + day + hours + minutes + seconds;
     }
     else if (dateformat.toLowerCase() == "yyyymmddhhmmssms") {
         dateStr = year + month + day + hours + minutes + seconds + minseconds;
     }
     else if (dateformat.toLowerCase() == "yymmddhhmmssms") {
         dateStr = year.toString().substring(2, 4) + month + day + hours + minutes + seconds + minseconds;;
     }
     else {
         dateStr = year + month + day;
     }
     return dateStr;  
 }
 
 //获取单据的序列号
 var GetSerialBillNo = function(num,length,format){
     //获取序号值
    var serialNum = GetSerialNum(num,length);
    //获取日期字符串
    var serialdate = getFullDataStr("yyyymmdd");
    //拼接对应单号
    var serialBillNo =serialdate+ serialNum;
    console.log(serialBillNo);
    return serialBillNo;
}

<body>
<button onClick="GetSerialNum(5,4)">获取序号值</button>
<button onClick="GetSerialBillNo(8,6,'yyyymmdd')">获取单据号</button>
</body>

根据上面输出的结果

0005
000008
20201201000008

注意：真正调用的时候，自己传递函数的参数值，记得把打印去掉

参考网址：

https://www.cnblogs.com/jokerjason/p/9355768.html